The present invention relates to zero halogen thermoplastic elastomer compositions based on olefinic polymers. The compositions have low smoke, low corrosivity, low toxicity, and low heat release properties, as well as favorable flame resistance and char integrity with non-dripping characteristics, while maintaining favorable electrical insulation properties.
The preferred composition may be employed as electrical insulation, insulation skin, jacket (sheath), buffer, cross web (spline), shield, and/or separator materials in the manufacture of optical fibers or electrical wires and cables.
A broad range of electrical conductors and electrical cables are installed in modern buildings for a wide variety of uses. Some of these uses are data transmission for computer systems, voice communications, as well as control signal transmission for building security, fire alarm, and environmental control systems. Some of these cables networks are frequently installed in the through-space between the dropped ceiling and floor above, currently referred to as the Plenum area. Electrical conductors and cables extending through Plenum areas are governed by special provisions of the National Electric Code (xe2x80x9cNECxe2x80x9d).
The National Fire Protection Association (xe2x80x9cNFPAxe2x80x9d) recognized the potential flame and smoke hazards created by burning cables in Plenum areas and adopted in the NEC a standard for flame retardant and smoke suppressed cables. This standard permits the use of cable without conduit, so long as the cable exhibits low smoke and flame retardant characteristics. The test method used for such distinctions is the Steiner Tunnel flame test. This method has been adopted as NFPA 262 and Underwriters Laboratory (UL) 910. The test conditions for are as follows: a 300,000 BTU/hour flame is applied for 20 minutes to a tray of cable, which is 1 foot wide by 24 feet long, mounted horizontal within the Steiner Tunnel. The criteria for passing the flame test is a maximum flame spread of five feet, a maximum optical density of smoke less than or equal to 0.50, and an average optical density of smoke of less or equal to than 0.15.
The most commonly used telecommunication cable has four unshielded twisted pairs of insulated conductors (4UTP). A majority of these cables use fluorinated ethylene/propylene polymers (FEP) as the insulation material of choice with a low smoke polyvinyl chloride jacket. The advantages of FEP are excellent electrical properties with a high degree of flame retardancy. However, FEP has many disadvantages as well. FEP is very expensive and drips off of the cable when exposed to a flame. This hot molten polymer also will release a very aggressive acid gas in hydrofluoric acid (HF). This gas is extremely toxic to humans when they are even exposed to small amounts.
An insulation material or combination of materials that could pass all the electrical and smoke requirements of NFPA 262, while not dripping off the cable when exposed to a flame, be low toxic and low corrosivity, and come at a lower cost would be useful in this application. This need has been established by industry trends of replacing some of the FEP with an alternate cheaper material. Until this time there has not been an insulation composition that was able to fully replace the FEP insulation in the telecommunications cable.
The present polymeric compositions can fully or partially replace FEP insulation while not dripping off the cable when exposed to a flame, with toxicity and corrosivity and comes at a much lower cost.
The present improved zero halogen polymeric compositions comprise:
(a) a base polymer selected from the group consisting of at least one polyolefin polymer;
(b) a filler comprising at least one substance selected from the group consisting of metal hydrates, oxides, carbonates, talcs, clays, molybdates, borates, stannates, graphite, carbon black, silicates and phosphates;
(c) an additive comprising at least one substance selected from the group consisting of an antioxidant, an organosilane, a pigments and a lubricant;
(d) an unsaturated dicarboxylic anhydride; and
(e) a nanoclay filler.
In preferred improved zero halogen polymeric compositions, the at least one polyolefin polymer is selected from the group consisting of polyethylene (PE) (including very low density polyethylene (VLDPE), low density polyethylene (LDPE), high density polyethylene (HDPE)), polypropylene (PP), ethylene-propylene copolymer (sometimes referred to as ethylene-propylene rubber (EPR)), ethylene-vinyl acetate (EVA), ethylene-ethyl acrylate (EEA), ethylene-methyl acrylate (EMA), copolymers containing ethylene monomeric units and terpolymers containing ethylene monomeric units. The at least one polyolefin polymer may be crosslinked or grafted by any convenient method, such as chemical crosslinking using organic peroxide or chemical grafting using acrylic and maleic acids.
In preferred improved zero halogen polymeric compositions, the unsaturated dicarboxylic anhydride is selected from the group consisting of a maleic anhydride, an itaconic anhydride, an aconitic anhydrides, a citraconic anhydride, and unsaturated dicarboxylic anhydrides modified thermoplastic polymers, where at least some of the anhydride functional groups are replaced by a functional group selected from the group consisting of carboxyl and carboxylate functional groups.
In preferred improved zero halogen polymeric compositions, the nanoclay filler is selected from the group consisting of synthetic silicate montmorillonites, natural layered silicate montmorillonites (the main fraction of the clay mineral bentonite) and a layered alumna-silicate.
In the preferred nanoclay filler, the individual platelets are approximately 1 micron in diameter, giving them an aspect ratio of about 1000:1. It is this morphology that leads to increased barrier properties with respect to moisture, resistance of the composition to deformation, resistant of the composition to whitening and/or blooming, improved mechanical strength (tensile modulus and flexural modulus), sizeable drop in heat release rate and smoke emission properties, and improved flame retardancy of the polymeric compositions. The nanoclay fillers are preferably chemically modified to increase the hydrophobicity of their surfaces.
Improved insulated wires comprise a wire and at least one polymeric composition, as defined above, surrounding the wire. Improved insulated conductors comprise a plurality of bunched, twisted, and/or bundled insulated wires, as defined above.